get rid of threads in the scoped_timer thread pool prior to forking, on non-Windows (#4833)

* on POSIX systems, fork() is dangerous in the presence of a thread
pool, because the child process inherits only the thread from the
parent that actually called fork().

this patch winds down the scoped_timer thread pool in preparation for
forking; workers will get freshly created again following the fork
call.

src/util/scoped_timer.cpp

@@ -29,6 +29,9 @@ Revision History:
 #include <mutex>
 #include <thread>
 #include <vector>
+#ifndef _WINDOWS
+#include <pthread.h>
+#endif
 
 struct scoped_timer_state {
     std::thread * m_thread { nullptr };
@@ -119,6 +122,16 @@ scoped_timer::~scoped_timer() {
     dealloc(m_imp);
 }
 
+void scoped_timer::initialize() {
+#ifndef _WINDOWS
+    static bool pthread_atfork_set = false;
+    if (!pthread_atfork_set) {
+        pthread_atfork(finalize, nullptr, nullptr);
+        pthread_atfork_set = true;
+    }
+#endif
+}
+
 void scoped_timer::finalize() {
     unsigned deleted = 0;
     while (deleted < num_workers) {
@@ -138,4 +151,5 @@ void scoped_timer::finalize() {
             delete w;
         }
     }
+    num_workers = 0;
 }

src/util/scoped_timer.h

@@ -26,5 +26,10 @@ class scoped_timer {
 public:
     scoped_timer(unsigned ms, event_handler * eh);
     ~scoped_timer();
+    static void initialize();
     static void finalize();
 };
+
+/*
+    ADD_INITIALIZER('scoped_timer::initialize();')
+*/